This invention relates to a phosphor adapted to carry out luminescence by excitation due to impingement of electrons accelerated under an acceleration voltage of hundreds of volts to about 2 kV thereon and a method for preparing the same, and more particularly to a phosphor highly suitable in use for a luminous display section of each of various luminous elements such as, for example, a display device using a field emission cathode, a write head for a printer based on a principle of the fluorescent display device, a fluorescent display device of high luminance suitable for use in a display panel of the type mounted on a vehicle, and the like.
A conventional fluorescent display device is generally adapted to be driven under an anode voltage of a relatively low level such as 100 V or less. Driving of the fluorescent display device under such a low anode voltage causes a depth by which electrons accelerated under the low anode voltage enter into a phosphor to be as small as tens of angstroms, resulting in a region of the phosphor excited for luminescence being restricted to such a small depth.
Therefore, doping of an additive acting as a luminous center which has been conventionally carried out with respect to a phosphor excited for luminescence by electrons accelerated under such a low anode voltage is limited to a region of the phosphor which has such a reduced depth. Unfortunately, doping of the additive which is concentratedly carried out with respect to such a shallow region of the phosphor causes a crystal lattice defect to occur in the phosphor in such a manner that the number of lattice defects is proportional to the number of particles of the additive doped. FIG. 4 shows the relationship between a depth of doping of an additive or dopant in the phosphor and a concentration of each of the dopant and the lattice defect. As will be noted from FIG. 4, a distribution of concentration of the additive or dopant has a shape similar to that of the lattice defect, except that the latter is deviated toward a surface of the phosphor as compared with the former. This indicates that the lattice defect concentratedly occurs in a region of the phosphor in proximity to the surface thereof, thereby failing to permit the phosphor to exhibit sufficient luminance.